Insulin, islet amyloid polypeptide and C-peptide interactions evaluated by mass spectrometric analysis

Zinc and pH modulate the ability of insulin to inhibit aggregation of islet amyloid polypeptide

Aggregation of the human islet amyloid polypeptide (hIAPP) contributes to the development and progression of Type 2 Diabetes (T2D). hIAPP aggregates within a few hours at few micromolar concentration in vitro but exists at millimolar concentrations in vivo. Natively occurring inhibitors of hIAPP aggregation might therefore provide a model for drug design against amyloid formation associated with T2D. Here, we describe the combined ability of low pH, zinc, and insulin to inhibit hIAPP fibrillation. Insulin dose-dependently slows hIAPP aggregation near neutral pH but had less effect on the aggregation kinetics at acidic pH. We determine that insulin alters hIAPP aggregation in two manners. First, insulin diverts the aggregation pathway to large nonfibrillar aggregates with ThT-positive molecular structure, rather than to amyloid fibrils. Second, soluble insulin suppresses hIAPP dimer formation, which is an important early aggregation event. Further, we observe that zinc significantly modulates the inhibition of hIAPP aggregation by insulin. We hypothesize that this effect arose from controlling the oligomeric state of insulin and show that hIAPP interacts more strongly with monomeric than oligomeric insulin.

Undercover Toxic Ménage à Trois of Amylin, Copper (II) and Metformin in Human Embryonic Kidney Cells

In recent decades, type 2 diabetes complications have been correlated with amylin aggregation, copper homeostasis and metformin side effects. However, each factor was analyzed separately, and only in some rare cases copper/amylin or copper/metformin complexes were considered. We demonstrate for the first time that binary metformin/amylin and tertiary copper (II)/amylin/metformin complexes of high cellular toxicity are formed and lead to the formation of aggregated multi-level lamellar structures on the cell membrane. Considering the increased concentration of amylin, copper (II) and metformin in kidneys of T2DM patients, our findings on the toxicity of amylin and its adducts may be correlated with diabetic nephropathy development.

Glycated Insulin Exacerbates the Cytotoxicity of Human Islet Amyloid Polypeptides: a Vicious Cycle in Type 2 Diabetes

The aggregation of human islet amyloid polypeptide (hIAPP) is one of the triggering factors of type 2 diabetes mellitus (T2DM). hIAPP is cosynthesized, costored, and cosecreted with insulin in pancreatic β-cells, and insulin inhibits hIAPP aggregation. In T2DM patients, long-term hyperglycemia causes glycation of near 10% of total insulin. The glycation not only modifies insulin but also cross-links insulin into oligomers. However, the effect of glycated human insulin on hIAPP aggregation is unknown. In this study, four physiologically relevant monosaccharides, methylglyoxal, glucose, fructose, and ribose were used to glycate human insulin and two C-terminus truncated insulin analogues. Glycated insulin monomers or low molecular weight oligomers such as dimers significantly exacerbated the cytotoxicity of hIAPP. Notably, glycation-induced cross-linking of insulin inhibited the aggregation, membrane disruption, and cytotoxicity of hIAPP, which was corroborated by a control study using EGS-induced cross-linking of insulin or lysozyme. Removal of B29^Lys on the C terminus of the insulin B chain not only abolished glycation-induced cross-linking but also attenuated the aggravation effect of glycated insulin on hIAPP cytotoxicity. Taken together, this study reveals a vicious cycle in T2DM, that hyperglycemia-driven insulin glycation exacerbates the cytotoxicity of hIAPP, which accelerates β-cells death and further deteriorates T2DM.

Zinc in Pancreatic Islet Biology, Insulin Sensitivity, and Diabetes

About 20 chemical elements are nutritionally essential for humans with defined molecular functions. Several essential and nonessential biometals are either functional nutrients with antidiabetic actions or can be diabetogenic. A key question remains whether changes in the metabolism of biometals and biominerals are a consequence of diabetes or are involved in its etiology. Exploration of the roles of zinc (Zn) in this regard is most revealing because 80 years of scientific discoveries link zinc and diabetes. In pancreatic β- and α-cells, zinc has specific functions in the biochemistry of insulin and glucagon. When zinc ions are secreted during vesicular exocytosis, they have autocrine, paracrine, and endocrine roles. The membrane protein ZnT8 transports zinc ions into the insulin and glucagon granules. ZnT8 has a risk allele that predisposes the majority of humans to developing diabetes. In target tissues, increased availability of zinc enhances the insulin response by inhibiting protein tyrosine phosphatase 1B, which controls the phosphorylation state of the insulin receptor and hence downstream signalling. Inherited diseases of zinc metabolism, environmental exposures that interfere with the control of cellular zinc homeostasis, and nutritional or conditioned zinc deficiency influence the patho-biochemistry of diabetes. Accepting the view that zinc is one of the many factors in multiple gene-environment interactions that cause the functional demise of β-cells generates an immense potential for treating and perhaps preventing diabetes. Personalized nutrition, bioactive food, and pharmaceuticals targeting the control of cellular zinc in precision medicine are among the possible interventions.

Insights into the binding sites of sulforaphane on insulin studied by electrospray ionization mass spectrometry

RATIONALE

Sulforaphane (SFN) is a natural isothiocyanate, known to reduce the risk of cancer and also aortic damage and diabetic cardiomyopathy induced by type 2 diabetes, etc. A more detailed knowledge on the direct interaction of SFN with insulin and its binding sites is necessary for better understanding the role of SFN on diabetes.

METHODS

Liquid chromatography/electrospray ionization mass spectrometry (LC/ESI-MS) and in-source fragmentation experiments were performed on a Thermo Exactive orbitrap mass spectrometer. The solution of insulin and SFN was incubated and analyzed by mass spectrometry. Isotopic distribution pattern, accurate mass values and theoretical product ions were used to analyze the mass spectrometry data. The nature of binding of SFN and its binding sites with insulin were evaluated by LC/MS data.

RESULTS

ESI-MS analysis of the incubated solution of insulin and SFN showed 1:1 and 1:2 complexes of [Insulin-SFN]. LC/MS analysis revealed that the [Insulin+SFN] complexes were due to covalent binding of SFN at two different sites. The in-source fragmentation experiments revealed that the SFN is binding to the NH2 groups of N-terminal amino acids of A and B chains of insulin. Further study of SFN with insulin reduced with dithiothreitol (DTT) showed exclusive modification of cysteines with SFN.

CONCLUSIONS

The interaction of SFN was studied with insulin using ESI-MS. SFN is found to bind covalently with the free NH2 group of the N-terminal of the A and B chains of insulin. However, when insulin is reduced SFN preferably binds to SH groups of cysteines. Hence, the present study helps in the understanding of the binding sites of SFN on insulin.

Conformational dynamics of α-synuclein: insights from mass spectrometry

Different mass spectrometry approaches are combined to investigate the conformational flexibility of α-synuclein.